The present invention relates to an image forming apparatus for forming a toner image using the electrophotographic process, an image fixing device for fixing the toner image, and an image forming system equipped with the image forming apparatus and image fixing device, particularly to a technique for preventing the fixing roller surface from being roughened.
In an image forming apparatus using the electrophotographic process, a toner image is formed on an image carrier of a rotating photoreceptor drum and photoreceptor belt, and the formed toner image is directly or indirectly transferred onto a recording sheet The image is further fixed so as to form an image thereon.
In the aforementioned image forming process, the toner image transferred electrostatically onto the recording sheet is fixed on the recording sheet in a stable state by heat and pressure of a fixing roller. Incidentally, image formation (fixing) is repeated using the recording sheet of one and the same size, and side ends of the recording sheet is brought in contact with the same positions of the fixing roller surface. This may cause a scratch or abrasion on the fixing roller surface, with the result that the service life of the fixing roller will be reduced.
Further, uniform fixing will be interfered by such a scratch and abrasion, and image quality will be deteriorated. Such a problem arises conspicuously when a transfer sheet of a certain size has been repeatedly used to produce a scratch or abrasion on the fixing roller, and a recording sheet having a size greater than that of the transfer sheet is used.
The following patent documents have been proposed in an effort to avoid such a scratch or abrasion (hereinafter referred to as “roughness”) on the fixing roller surface.
In the Unexamined Japanese Patent Application Publication No. 2008-298925, the end of the recording sheet is sandwiched on the upstream side of the fixing device, whereby the thickness of the end is reduced. This arrangement reduces the amount of the fixing roller surface from being scratched by the end of the recording sheet sandwiched and reduced in thickness.
The Unexamined Japanese Patent Application Publication No. 2006-317881 discloses a technique of reducing the level of roughness on the surface by introducing an abrasive agent for polishing the surface of the fixing belt.
The Unexamined Japanese Patent Application Publication No. 2008-225276 provides the fixing roller or fixing belt surface with a recessed section for reducing the pressure.
In the Unexamined Japanese Patent Application Publication No. 2007-34068, the contact member of a belt is arranged in a fixing device constituting an upper belt and a lower roller to ensure that the surface roughness of the lower roller is greater than that of the upper belt.
Incidentally, an image forming apparatus has been proposed in recent years wherein a second fixing device is provided on the downstream side of the fixing device to re-melt the fixed toner, whereby the level of glossiness is upgraded. This second fixing device is intended to upgrade the level of glossiness of the toner image. Accordingly, the proposals given in the aforementioned patent documents cannot be used directly. Further, this proposal has failed to introduce any technique suited to the second fixing device, capable of reducing the level of roughness on a fixing roller surface.
Further, even when image formation (fixing) is repeated using the recording sheets of the same size, the conveying conditions differ according to the type of the sheets. This may result in a slight difference in the positions to be passed through by sheets between the first fixing roller and the second fixing roller. Thus, although there may be no problem with the first fixing roller, the roughness of the surface on the part of the second fixing roller may interfere with the uniform glossiness.